Reversible addition-fragmentation chain transfer (RAFT) polymerisation, as described in International Patent Publication No. WO 98/01478, is a polymerisation technique that exhibits the characteristics associated with living polymerisation. Living polymerisation is generally considered in the art to be a form of chain polymerisation in which irreversible chain termination is substantially absent. An important feature of living polymerisation is that polymer chains will continue to grow while monomer and the reaction conditions to support polymerisation are provided. Polymers prepared by RAFT polymerisation can advantageously exhibit a well defined molecular architecture, a predetermined molecular weight and a narrow molecular weight distribution or low polydispersity.
RAFT polymerisation is believed to proceed under the control of a RAFT agent according to a mechanism which is simplistically illustrated below in Scheme 1.

With reference to Scheme 1, R represents a group that functions as a free radical leaving group under the polymerisation conditions employed and yet, as a free radical leaving group, retains the ability to reinitiate polymerisation. Z represents a group that functions to convey a suitable reactivity to the C═S moiety in the RAFT agent towards free radical addition without slowing the rate of fragmentation of the RAFT-adduct radical to the extent that polymerisation is unduly retarded. The ability for both R and Z to function in this way for a given agent is known to be influenced by the nature of the monomer to be polymerised and the polymerisation conditions.
In practice, the R and Z groups of a RAFT agent for use in a given polymerisation reaction are typically selected having regard to the type of monomers that are to be polymerised. For example, it is known in the art that Z groups that afford dithiocarbamate and xanthate RAFT agents can in general be used for controlling the polymerisation of monomers that produce relatively unstabilised propagating radicals (i.e. less activated monomers such as vinyl acetate, N-vinylpyrrolidone and N-vinylcarbazole), whereas Z groups that form dithioester and trithiocarbonate RAFT agents can in general be used for controlling the polymerisation of monomers that produce relatively stabilised propagating radicals (i.e. more activated monomers such as methacrylate esters and styrene). In other words, a given RAFT agent will generally be unsuitable for use in controlling the polymerisation of both less activated and more activated monomers (i.e. monomers having markedly disparate reactivities e.g. styrene and vinyl acetate).
Having said this, under limited circumstances a RAFT agent may be used to polymerise a mixture of less activated and more activated monomers. In that case, those skilled in the art will appreciate that provided the reactivity and mole ratios of the selected monomers are suitable, a RAFT agent may be used to prepare a statistical (or random) copolymer comprising the polymerised residues of less activated and more activated monomers. Those skilled in the art will also appreciate that one practical upshot of this is that to date it has proven difficult to prepare by RAFT polymerisation block copolymers derived from less activated and more activated monomers (e.g. a vinyl acetate-acrylate block copolymer).
It should also be noted that WO 2006/122344 discloses RAFT agents in which Z=F. RAFT agents of this type are said to have the potential to polymerise monomers having disparate reactivities. However, such RAFT agents are generally difficult to synthesise, they may be unstable under certain polymerisation conditions, and their effectiveness has yet to be proven.
Although there are numerous advantages to be gained by employing conventional methods for polymerising monomers under the control of a RAFT agent to form polymer, it would be nonetheless desirable to provide a RAFT polymerisation method and RAFT agents that offered further utility relative to those currently known.